shells_path is now a directory of .shell files

Matches the existing units_path pattern — shells_path points to a
directory that is scanned for .shell files rather than a single
monolithic definitions file.

sample/rex.config

@@ -3,7 +3,7 @@
 	"project_root": 	"$HOME/development/internal/rex/sample",
   	"units_path":		"units/",
   	"logs_path": 		"logs/",
-	"shells_path":		"shells/shells.definitions",
+	"shells_path":		"shells/",
   	"config_version": "5"
   }
 }

sample/shells/bash.shell

@@ -5,12 +5,6 @@
 			"path": 		"/usr/bin/bash",
 			"execution_arg": 	"-c",
 			"source_cmd":		"source"
-		},
-		{
-			"name":			"ksh",
-			"path":			"/usr/bin/ksh",
-			"execution_arg":	"-c",
-			"source_cmd":		"source"
 		}
 	]
 }

sample/shells/ksh.shell

@@ -0,0 +1,10 @@
+{
+	"shells":	[
+		{
+			"name":			"ksh",
+			"path":			"/usr/bin/ksh",
+			"execution_arg":	"-c",
+			"source_cmd":		"source"
+		}
+	]
+}